Why didn't the alien just make the spaceship if it could make it in 10 seconds at the end of Super 8?

He was captured trying to fix his ship. He stole car engines and
microwaves and whatever. He spend so much time underground building
all that techno-looking stuff. And at the end he just sucks everything
metal together and sucks the metal cubes together and it becomes a
spaceship in 10 seconds. So why didn't he just suck everything
together before he got captured?